Traditional methods of blasting or breaking rock in quarries and mines make use of high energy explosives, often referred to as detonating explosives. High energy explosives crush and pulverise the rock which can then be removed for either retrieving a sought after mineral within the rock or for disposal of the rock or both.
The problem with detonating explosives is that the ignition of the explosive is followed by a violent shockwave which may cause rock fragments to be projected from the explosion site. The projected rock fragments pose a great risk to mine workers, thus commonly requiring a large area surrounding the blasting site to be cleared. Furthermore, the pulverisation of the rock may create a dense cloud of dust surrounding the blasting site, making it impossible to work at the site for extended periods of time.
The problems associated with traditional methods of blasting or breaking rock resulted in the development of rock breaking explosives commonly referred to as non-detonating explosives. Non-detonating explosives function by containing and directing rapidly expanding gases within and against the rock, thereby causing the rock to break without a violent shock wave and pulverisation of the rock.
Non-detonating explosives are used by drilling boreholes into the rock, inserting non-detonating explosive cartridges containing a gas generating compound, commonly a propellant, into the boreholes and igniting the cartridges. Prior to ignition of the cartridge, the borehole must be stemmed commonly by packing particulate material, usually sand, into the borehole after insertion of the cartridge. The packed particulate material keeps the gases created by the cartridge within the borehole once the cartridge has been ignited resulting in high pressure being created within the borehole.
A drawback of non-detonating explosives is that adequate stemming of the borehole is of utmost importance, failure of which may cause some of the gas to escape thereby reducing the pressure exerted on the rock and causing the cartridge to be less effective. Furthermore, stemming of boreholes that run at a downward slope may be difficult thus often being very time consuming to achieve. Also, stemming material needs to be transported to the blasting site.
A self-stemming cartridge is proposed in U.S. Pat. No. 8,342,095. One embodiment of the cartridge disclosed in the patent has a sheath which is tapered radially inwardly at one end and which houses a gas generating compound and a cone. The patent discloses that the cone is forced in the direction of the taper upon ignition of the gas generating compound and forces the sheath outwardly, thereby stemming the borehole.
Drawbacks of the disclosed cartridge include the cartridge having a plug at one end which will be ejected from the cartridge prior to stemming, thus causing the stemming operation to stop and the cartridge to be ejected from the borehole without breaking any of the rock.
Furthermore, the sheath is of a solid construction. This will permit gas to escape about the periphery of the cone when the sheath flexes outwardly after ignition and from the gas pressure within the cartridge. It is thus highly unlikely that the cone will operate to expand the sheath. Also, such flexing will cause the development of empty pockets within the sheath into which the gas can move, thus causing a drop in pressure within the cartridge and resulting in a cessation of combustion of the gas generating compound.
A further disadvantage of the cartridge disclosed in U.S. Pat. No. 8,342,095 is that the sheath is a solid tube and thus unlikely to expand sufficiently to stem the hole. Also, the detonator cord runs between cone and sheath creating a gap which will permit gas to escape and thus prevent proper working of the cartridge during manufacture and handling. The gap will also permit the propellant to leak out of the cartridge. Furthermore, the detonator must be inserted into the cartridge before it can be filled with propellant. This will create an inherently dangerous situation during assembly as there is a possibility of the detonator igniting the propellant during assembly.
There is no evidence of the cartridges proposed by U.S. Pat. No. 8,342,095 being commercially available and the applicant believes this to be a result of these not being capable of functioning for the reasons given above.
WO 2013/150462 discloses a self-stemming cartridge which includes a piston movable relative to a static member under pressure of gas generated in a cartridge to cause expansion of the static member and stemming of the borehole. This cartridge is effective yet requires moving parts to operate.
In this specification, “propellant” shall have its widest meaning and include any suitable gas producing material, and “igniter” shall mean any device capable of causing the propellant to produce gas.